I have a co-worker that is always saying "Therefore, A B C" when the "A B C" isn't a conclusion from any sort of deductive reasoning. For example, Me: ... thus, that's how it works. Her: I ...
I was taught that, at least, 'therefore' and 'so' and can be used interchangeably, one being informal, the other formal. But, even when written, replacing 'so' with 'therefore' doesn't seem correct. ...